Hello Ktchiu,
Welcome to the community!
Do you receive any error message when you try to send a MMS to China or does your recipient simply not receive the picture?
If you're able to send and/or receive MMS locally, then your phone settings would be correct. There are no specific settings for sending a MMS internationally. The only consideration would be the potential additional fees to send MMS internationally.
The issue of your recipient not receiving a MMS you sent would be on their network and not with Fido. There have been others who have also had troubles sending MMS to China (see my previous post here). At the time, I questioned whether China might be blocking all foreign MMS from entering the country. That said, China Mobile and China Unicom are listed as selected destinations for picture and video messaging (see here). Do you know your recipient's mobile provider?
Even if your recipient uses either China Mobile or China Unicom, there still is the possibility that China is actively blocking foreign MMS. Your best option is for your recipient to inquire on their end.
**edit** Something else to consider is the maximum size limits may be different. At least for China Mobile (HK) the max size for an image is around 290Kb in jpeg or gif format (see here). If you're trying to send a larger image, it might not go through. You might be able to limit the size of picture you're sending in your MMS settings**
**edit** Additionally, is your recipient's account set up to receive international MMS? Again, at least for China Mobile (HK), it appears there may be an additional MMS download fee for international MMS (see here). I'm not sure when that fee would be applicable since they also have separate data roaming charges. It would be best for your recipient to inquire with their provider**
Hope this helps 
Cheers
